1. Field of the Invention
This invention relates to an electronic video camera.
2. Description of the Related Art
Electronic video camera systems include systems called still video camera systems, which are arranged to convert an image of an object of shooting (hereinafter referred to as an object image) into an electrical signal at an image pickup part, to record the electrical signal on a suitable recording medium such as a two-inch floppy disk, a hard disk, an IC memory card or the like and to reproduce the object image recorded.
Generally, electronic video camera systems of the above-stated kind are arranged to have recourse to a flash device in taking a shot of an object under a dark condition. However, the reachable distance of the light of the flash device is limited. To compensate for the insufficiency of light, the image pickup part has recently come to be provided with a circuit for increasing the image pickup sensitivity of the image pickup part. The image pickup (shooting) sensitivity can be increased by operating a sensitivity increasing switch.
The sensitivity increasing circuit is arranged to increase the gain of an electric circuit system, so that the output level of an image sensor can be greatly amplified before it is outputted even in cases where the output level of the image sensor is low.
Some of the electronic video camera systems is provided with a high image quality switch for selecting a high image quality mode. In this case, a shooting action is performed by suppressing the gain of an electric system. With the electronic video camera system set in the high image quality mode, the S/N ratio of an image signal can be prevented from deteriorating if the quantity of light incident on the image sensor is sufficient.
Further, the electronic video camera systems are arranged in general to make white balance adjustment in recording and reproducing color image signals. The white balance adjustment is carried out by adjusting the gains of the R, G and B (three primary colors of light) components of an image signal outputted, for example, from the image pickup part according to the color adaptation of the human eye.
With the conventional electronic video camera system provided with the sensitivity increasing switch for increasing the image pickup sensitivity as mentioned above, however, a noise component generated by the image sensor is amplified along with the image signal. Then, the S/N ratio of the image signal is deteriorated as a whole by the noise component to degrade the image quality.
Another problem with the conventional electronic video camera systems lies in that, when the high image quality mode is selected by operating the high image quality mode switch, efficiency in utilizing the light of the object image becomes lower. The lower efficiency then lowers the image pickup sensitivity.
It is a further problem with the conventional electronic video camera system that, the gain of the blue color (B) component must be greatly increased if the color temperature of the object image is very low, or the gain of the red color (R) component must be greatly increased if the color temperature is high. In either case, the S/N ratio of the image signal is deteriorated by the increased gain to eventually degrade the image quality.
During recent years, some of electronic video cameras have come to be arranged to convert the object image into an electrical signal by a photoelectric conversion element and then to convert the electrical signal further into a digital signal before recording. The cameras of this kind digitally store image signals by using a memory element such as an IC memory card or a hard disk or the like as a recording medium.
Like in the case of silver-halide type cameras, the operability of electronic video cameras of the above-stated kind can be greatly improved by arranging them to have a full automatic shooting mode in which a shutter speed, an exposure, the use or nonuse of a flash device, etc., are automatically decided to lessen the burden of the camera operator at the time of shooting.
However, unlike the full automatic shooting mode of the silver-halide type camera, the electronic video camera involves its own parameters such as a compression ratio for the image signal, etc., even while the shutter speed, an exposure and the use or nonuse of a flash device are automatically decided. Therefore, even if the control method of the silver-halide type camera for the full automatic mode is applied to the electronic video camera, it has been hardly possible to sufficiently improve the operability of the electronic video camera by enabling it to be completely in the full automatic shooting mode.
The performance of the image sensor which serves to convert the object image into an electrical signal is being improved. Generally, however, the sensitivity of the image sensor still remains low. It is thus hardly possible to obtain a sufficiently sharp image signal by taking a shot under a low illuminance condition without any auxiliary illumination. To solve this problem, some of the known electronic video cameras is arranged to be capable of altering the sensitivity of an image pickup part (consisting of an image sensor and an amplifier for amplifying the output of the image sensor) and to take a shot under a low illuminance condition by increasing the sensitivity of the image pickup part.
However, as mentioned above, the image quality of the image signal degrades in general when the image pickup sensitivity is increased. The image signal having such a degraded image quality has been recorded in the same manner as a normal image signal which has a good image quality and is obtained by shooting without increasing the image pickup sensitivity. In other words, the conventional electronic video camera has been arranged to allow each recording signal recorded on a recording medium to have a predetermined amount of information irrespective of the image quality. Such arrangement has resulted in a poor image signal recording efficiency.